1. Field of the Invention
The present invention relates in general to a balanced input/output filter device. In particular, the present invention relates to a ceramic-laminated planar filter comprising balanced input/output terminals.
2. Description of the Related Art
Generally speaking, surface acoustic wave filters are important band-pass filter devices in communications. Although the surface acoustic wave filters have good frequency responses in rejection band, they have disadvantages of cost and size. Therefore, laminated planar filters are employed to decrease the size of products and the cost of production.
A balanced-input-output configuration of laminated planar filters suitable for next-generation super-compact portable telephones is disclosed in 1999 IEEE MTT-S Digest from pages 1143 to 1146. FIG. 1 depicts an equivalent circuit diagram of a conventional two-order balanced input/output band-pass filter. A conventional two-order balanced input/output band-pass filter comprises a positive input terminal 111 and a negative input terminal 112, a positive output terminal 121 and a negative output terminal 122, and ceramic resonators 13 and 14. The ceramic resonators 13 and 14 are coupled between the positive input terminal 111 and a negative input terminal 112 and between the positive output terminal 121 and a negative output terminal 122, respectively. In addition, a conventional two-order balanced input/output band-pass filter further comprises six coupling capacitors 15, 16, 17, 18, 19, and 20. Capacitors 15, 19, 17 are coupled between the positive input terminal 111 and the positive output terminal 121, and capacitors 16, 20, 18 are coupled between the negative input terminal 112 and the negative output terminal 122.
FIG. 2 depicts the ceramic-laminated planar structure of the circuit in FIG. 1. As shown in FIG. 2, parallel metal layers (15xcx9c20) constitute capacitor structures, and resonators are composed by transmission lines 13 and 14. The transmission lines 13 and 14 are series-connected to the parallel metal layers and are shorter than a quarter wavelength.
FIG. 3 depicts a frequency response plot of the ceramic laminated planar structure in FIG. 2. As shown in FIG. 3, the frequency response of the rejection band of conventional two-order balanced input/output band-pass filter is poor. Therefore, the conventional balanced input/output band-pass filter cannot eliminate signals with undesired frequency.
To overcome this disadvantage, the conventional method is to add the orders of the conventional balanced input/output band-pass filter. Therefore, the conventional two-order balanced input/output band-pass filter circuit must add more ceramic resonators and coupling capacitors to improve the effect of filtering noise.
However, the approach mentioned above uses more elements. Therefore, the cost of fabricating and the size of filter will both increase, which will limit the design of communication apparatus. Moreover, the increase in quantity of elements used increases the insertion loss, so the intensity of the signal to be transmitted will be decreased, worsening the efficacy of the communication apparatus.
The object of the present invention is to provide a balanced input/output filter circuit, under the configuration of a two-order balanced input/output bandpass filter circuit. In the present invention, both sides of the ceramic resonators are serially connected to a pair of inductors and capacitors, respectively. Therefore, the intensity of the signal in rejection band will attenuate effectively without increasing elements of the circuit; moreover, the purpose of decreasing insertion loss and the size of the units will be achieved.
To achieve the above-mentioned object, the present invention provides a balanced input/output filter device. The balanced input/output filter device according to the present invention comprises two resonators, a pair of coupling capacitors, a positive input terminal and a negative input terminal, and a positive output terminal and a negative output terminal.
Both sides of one resonator are serial connected to spiral-type inductors on the other metal layers, and both sides of the other resonator are serial connected to parallel-type capacitors on the other metal layers. The coupling capacitors couple between the spiral-type inductors and the parallel-type capacitors respectively, and the output and input terminals are coupled to the spiral-type inductors and parallel-type capacitors respectively, which are coupled to the resonators.